We propose to study the mechanisms of activation of human T cells. We will focus on the study of the early events that follow binding of ligands that lead to interleukin 2 receptor (IL2r) expression and on the study of the nature of the interactions between T cells and monocytes that lead to IL2 synthesis and proliferation. The role of GTP binding (G) proteins in signal transduction that leads to the activation of phospholipase C (PLC), turnover of inositol phospholipids (Pt Ins) and translocation of protein kinase C (PKC) will be examined in T cells stimulated with anti CD3 monoclonal antibodies (mAb). A defect in T cell receptor coupled G protein will be ascertained in a patient with a signal transduction defect. Assays for G protein enhanced activation of PLC in cell membranes and reconstitution experiments on cell lines from the patient with signal transduction defect will be used to monitor the purification of receptor/CD3 coupled G protein from normal lymphocytes. We will study the role of Pt Ins, PKC activation, CD3 phosphorylation and rise in intracellular calcium (Ca++)i in different pathways of T cell activation. The relationship between the generation of the Pt Ins breakdown product inositol (1,4,5) triphosphate (IP3) and (Ca++)i rise will be examined in resting T cells stimulated with CD3 mAb. We will test the hypothesis that cell-cell interaction between ligand activated T cells and monocytes is necessary to induce IL1 responsiveness in T cells leading to IL2 synthesis. We will examine the role of LFA1 molecules in T cell activation, attempt to identify LFA1 receptors by raising the mAb to Epstein-Barr transformed B cells from LFA1 deficient patients and study the mechanism of LFA1-LFA1 receptor interaction. The role of these interactions in inducing IL1 responsiveness in T cells will be studied, and we will examine whether this responsiveness is due to induction of IL1 receptors, and results in IL2 synthesis allowing proliferation to proceed. The basic significance of this work is to define the events and pathways that lead to the elicitation of T cell mediated immunity. The results of the proposed studies will be applicable to the understanding and treatment of immunodeficiencies, autoimmune diseases and transplantation.